Evaluating the price of tiny kinetic mixing

We consider both "bottom-up" and "top-down" approaches to the origin of gauge kinetic mixing. We focus on the possibilities for obtaining kinetic mixings ϵ which are consistent with experimental constraints and are much smaller than the naive estimates (ϵ∼10-2-10-1) at the one-loop level. In the bottom-up approach, we consider the possible suppression from multiloop processes. Indeed we argue that kinetic mixing through gravity alone, requires at least six loops and could be as large as ∼10-13. In the top-down approach we consider embedding the Standard Model and a U(1)X in a single grand-unified gauge group as well as the mixing between Abelian and non-Abelian gauge sectors.